The main use one can imagine for this is adding tweak controls to
parts of a model that are already deformed by multiple other major
bones. It is natural to expect such locations to deform as if the
tweaks aren't there by default; however currently there is no easy
way to make a bone follow multiple other bones.

This adds a new constraint that implements the math behind the Armature
modifier, with support for explicit weights, bone envelopes, and dual
quaternion blending. It can also access bones from multiple armatures
at the same time (mainly because it's easier to code it that way.)

If this isn't saved/restored in DNA, prefer add a bConstraintTarget_Runtime struct which can have an owner.

Also note that it's questionable if we should even have this, such pointers can easily become invalid. There are many other planes in RNA that similar pointers would be convenient, but we've avoided it because keeping them valid is a hassle.